The present invention relates to a crystal growing process used for manufacture of a silicon single crystal or the like utilized as a semiconductor material.
A variety of methods are available for manufacturing silicon single crystals, of which the Czochralski process (the CZ process) is typical. In producing a silicon single crystal by the CZ process, as is well known, a seed is immersed in a silicon melt formed in a quartz crucible. The seed is gradually pulled up to allow a silicon single crystal to grow beneath the seed while rotating the crucible and the seed.
In pulling up a silicon single crystal by means of the CZ process, it is known that the defect distribution etc. in the crystal cross section are governed by the rate of crystal growth, therefore the pulling speed. More specifically, as the pulling speed is increased, a ring-shaped OSF generation zone is moved towards the periphery and is finally excluded to the outside of the effective portion of the crystal. Conversely, a decrease in the pulling speed drifts a ring-shaped OSF generation zone towards the central part of the crystal, and eventually the zone disappears in the central part.
While both the outside and inside of an OSF generation zone are defect generation zones, their kinds of defects are different. In addition, it is known that a significant increase of the pulling speed, as a matter of course, improves productivity while refining the defects. Consequently, a speed-up in the pulling has been pursued as an approach to growing crystals.
Provision of a heat shield is known as a technique for high speed pulling. A heat shield is a cylindrical heat shielding member in a shape of inverted truncated cone that is disposed surrounding the single crystal. The shield is provided to speed up the pulling by shielding the radiation heat primarily from the melt in the crucible and heaters placed outside the crucible to facilitate cooling of the single crystal to be pulled up from the melt.
Furthermore, attention is recently given to a technique where a cylindrical cooling member that is forcibly cooled by water is placed inside the heat shield (Japanese Patent Laid-Open Nos. 63-256593, 8-239291, 11-92272 and 11-292684) Installation of a cylindrical cooling member that is forcibly cooled by water inside the heat shield surrounding the single crystal facilitates cooling of the single crystal, particularly the high temperature portion thereof, leading to a further speedup of the pulling.
However, it has turned out that the conventional crystal growing process using a cooling member does not always permit the cooling member to effectively increase the pulling speed and also causes the problems to be described below with regard to the quality of the single crystal and the safety of operation.
A copper-based metal member forcibly cooled by water passage is frequently used as a cooling member from the viewpoint of cooling capacity and cost efficiency for the single crystal. Providing simply such a cooling member does not serve to increase the pulling speed in many cases. When the cooling member does not effectively act for a speedup of the pulling, the diffusion of heavy metals such as Fe and Cu is facilitated leading to contamination of the single crystal starting from its periphery, because the portion of the single crystal with a high temperature of 1300xc2x0 C. or above is elongated and the time in which the single crystal passes through an area of a high temperature of 1300xc2x0 C. or above is increased as well.
Speed up of the pulling requires cooling of the portion of the single crystal with a high temperature of 1300xc2x0 C. or above by a cooling member and its increased cooling capacity causes the risk of rapid cooling of the portion of 1300xc2x0 C. or below also. The rapid cooling of the portion of 1300xc2x0 C. or below also causes rapid deformation of the crystal caused by the cooling in dislocation of the crystal being pulled up. As a result, a residual stress is generated because of the difference of extent of the deformation in the boundary between the non-dislocated and dislocated portions, leading to the likely generation of cracks in cooling of the crystal drawing or after the pulling. When this crack is generated, the cooling member may be broken in some cases, leading to disasters including a steam explosion.
The object of the present invention to provide a crystal growing process that can make the cooling member effectively serve to increase pulling speed and also effectively prevent cracks caused by excessive cooling of the single crystal, in pulling the single crystal by the CZ process using a cooling member.
In order to achieve the above described object, the inventors focused attention on the dimensions of the cooling member and the surface temperature and have investigated in detail the relations between the high speed pulling and these factors. As a result, the following facts have been found.
When the thickness of the cooling member is less than 10 mm, the temperature of the cooling member is extremely increased because of the radiation from the single crystal, which in turn reduces the cooling efficiency for the crystal, leading to the difficulty in the realization of a high speed pulling. On the other hand, when the thickness exceeds 50 mm, not only the cooling efficiency stops increasing, but also the field of the view of a camera used for controlling the crystal diameter is restricted, and the flow of gases such as Ar that is flowed through the furnace is reduced leading to dislocation due to the precipitation of SiO.
When a portion with a temperature exceeding 500xc2x0 C. exists in the inner peripheral surface of the cooling member opposing to the outer peripheral surface of the single crystal to be pulled from the melt, the cooling efficiency for the crystal is dramatically reduced, leading to the unrealization of a high speed pulling.
When the height of the cooling member is less than 0.1 times the diameter of the crystal, the portion of the single crystal with a high temperature of 1300xc2x0 C. or above cannot be cooled efficiently. However, when its height exceeds 1.5 times the diameter of the crystal, the portion with a temperature of 1300xc2x0 C. or below is rapidly cooled, and when a dislocation is formed in the single crystal, cracks are likely to be generated.
When the distance from the lowest end of the cooling member to the melt exceeds 100 mm, the effect of cooling of the portion of the single crystal with a high temperature of 1300xc2x0 C. or above is significantly reduced, thereby making a speedup in the pulling difficult. On the other hand, when the distance is shorter than 10 mm, the risk of contact of the cooling member with the melt is increased. Further, the temperature of the melt close to the cooling member quickly decreases, causing the problem of the pulled crystal being likely to deform due to a decrease in the radial temperature gradient of the melt.
When the emissivity of the inner peripheral surface of the cooling member is less than 0.7, the effect of cooling of the portion of the single crystal with a high temperature of 1300xc2x0 C. or above is reduced, leading to a difficulty in speedup of the pulling.
When the flow rate of the cooling water supplied to the cooling member is less than 1.5 L/min, the temperature of the cooling member is increased, and therefore the crystal cannot be cooled effectively. When the flow rate exceeds 30 L/min, the effect of cooling of the crystal stops increasing, resulting in a waste of the cooling water.
The crystal growing process of the present invention is made in view of the above described findings and permits the cooling member to effectively serve to increase the pulling speed, while effectively preventing cracks caused by excessive cooling of the single crystal, by utilizing a cooling member with a height 1.5 times or less the diameter of the above described single crystal and also by keeping 500xc2x0 C. or below the temperature of the inner peripheral surface of the cooling member opposing to the outer peripheral surface of the single crystal, when crystal growing is carried out by utilizing a furnace equipped with a ring-shaped cooling member disposed surrounding the single crystal grown from a raw material melt by the CZ process.
The lower limit of the height of the cooling member is preferably 0.1 times or more the diameter of the single crystal, from the viewpoint of a speed-up in the pulling. The particularly preferable height is 0.25 to 1.0 times the diameter of the single crystal.
The temperature of the inner peripheral surface of the cooling member is not specified in particular, because the lower it is, the better. The particularly preferable surface temperature is 200xc2x0 C. or below.
For other factors than the height of the cooling member and the temperature of the inner peripheral surface, the thickness of the cooling member is preferably from 10 to 50 mm. Additionally, the flow rate of the cooling water to be supplied to the cooling member is preferably from 1.5 to 30 L/min. In addition, the distance from the lower end of the cooling member to the melt surface is preferably from 10 to 100 mm. Furthermore, the emissivity of the inner peripheral surface of the cooling member is preferably not less than 0.7.
The cooling member is made of a metal material which is forcibly cooled by water passage. The primary component of the metal is preferably copper, which exhibits good heat conduction and is inexpensive. When the cooling member is composed of a copper-based metal, blacking by plating with black Cr metal or oxidation is effective as a method of increasing the emissivity of the inner peripheral surface. In particular, plating with black Cr metal also has the effect of protecting soft copper-based metal to suppress contamination by the crystal of copper.
In general, the pressure of the atmosphere inside the furnace is not more than 13300 Pa, and so heat transfer by the gas can be neglected and the heat removal from the pulled crystal is exclusively by heat radiation. In the crystal growing process of the present invention the height of the cooling member and the surface temperature are specified from this viewpoint, and the distance from the crystal surface to the cooling member is not particularly specified because heat transfer by the gas can be neglected.
It is advisable that the cooling member be combined with a heat shield and placed inside of it. Combination with a heat shield not only facilitates the cooling of the crystal, but also effectively constrains a rise in the temperature of the cooling member itself, thereby increasing the pulling speed.